In use, an electromagnetic (EM) flowmeter relies on at least two measuring electrodes in order to measure the fluid velocity within a conduit such as a pipe to which the flowmeter is coupled. These electrodes are normally in contact with the flowing fluid. If this contact is lost due to, say, low or no fluid level in the flow meter, then flow measurement is no longer possible. Possible coating of the electrodes, or low fluid conductivity, may also affect the flowmeter accuracy. Consequently, measurement of the impedance of the measuring electrodes, and thus verification of electrode integrity, can be of great benefit. Such information is an important diagnostic and alarm source for EM flowmeters.
It is well established, for example in GB 2333161, to measure resistance between electrodes by injecting a constant current and measuring the resulting voltage to calculate the individual electrode impedances of an EM flowmeter.
Injecting an electrical signal onto each measuring electrode results in a voltage being developed which is related to the impedance of that electrode to the measuring fluid. The induced voltage, which may be small, can have an amplitude lower than the small amplitude flow related signal induced by the flowing fluid as a result of electromagnetic induction. This flow induced signal is used to determine the fluid flow velocity, and it is important that any interference to the flow signal from signal injection used to measure impedance be kept to a minimum.
One technique, often employed to minimise interference, is to disable or interleave the flow velocity measurement whilst an electrode impedance measurement is being made. Whilst effective, this degrades the signal-to-noise ratio of the resulting flow measurement, and discontinuous operation means valuable flow measurement data may be lost or not obtained.
Another technique employed by modern low power EM flowmeters, such as those powered for example by batteries, is to minimise the duration for which the EM flowmeter is active over a given period of time. For instance, to reduce power consumption it is common practice to, say, take one flow measurement reading every 15 seconds. Minimising the duration of the flow measurements over a given period of time has benefits in reducing average power consumption and, if the flowmeter is battery powered, extending battery life. However, interleaving flow velocity measurements with electrode impedance measurements, as discussed above, would result in the overall measurement cycle being extended, thereby increasing the power consumption of the flowmeter and, if it is battery powered, decreasing the overall period for which it can operate before the batteries need to be replaced or recharged.
There is therefore a desire to improve the efficiency with which in situ electrode impedance measurements can be made in respect of an electromagnetic flowmeter during use.
There is also a desire to enable the impedance measurements to be made in a manner that does not cause significant interference or degradation to the desired flow signal and, more generally, does not result in appreciable measurement inaccuracies in the measured fluid velocity.